The invention relates to a drift tube for an ion mobility spectrometer which is assembled from stacked tube segments and which has electrodes that generate an electric drift field and are electrically contactable from the outside of the drift tube.
Ion mobility spectrometry is a method for the highly sensitive detection of foreign substances of low concentration in ambient air or other gases. The method can be implemented as a comparatively compact arrangement and with a simple technical set-up. It is therefore particularly suited for use in portable gas analyzers and warning devices. Ion mobility spectrometry has been elucidated in U.S. Pat. No. 3,699,333 (Cohen et al.) and Pat. No. 4,777,363 (Eiceman), for example.
In ion mobility spectrometry, it is generally the mobility of ions or charged clusters in gases under the influence of an electric field which is analyzed. The mobility, taking into account gas temperature and pressure, is characteristic of the ions or charged clusters.
The most common type of ion mobility spectrometer being used at present is one with a drift tube of the time-of-flight type. In the reaction region of this type of drift tube, gas molecules are normally partially ionized by radioactive materials such as 63Ni. The ionization of the substances to be analyzed from a fed-in gas sample is typically carried out only in subsequent chemical reactions, which are influenced by gaseous water or introduced gaseous doping agents such as ammonia. In the reaction region, the ions of the substances to be analyzed drift in an axial electric drift field to a gating mechanism, e.g. a Bradbury-Nielsen shutter or a potential barrier. By briefly switching off or switching over potentials across the gating mechanism, ions are admitted as a pulsed ion current into the drift path of the drift tube, where they move in an axial electric drift field to the collecting electrode. The ion-specific mobility means that the ionic species exhibit different speeds and are temporally separated. The time delay of the ion current at the collecting electrode with respect to the opening of the gating mechanism determines the ion-specific drift times, which are a measure of the mobility of the ions. The reaction region and the drift path together form the drift region of the drift tube, in which the ions drift along an axial electric drift field in the direction of the collecting electrode.
In other types of ion mobility spectrometer such as the filter types (FAIMS=“Field Asymmetric Ion Mobility Spectrometer”) or the multi-electrode types (aspiration types), the ions drift in electric fields aligned radially or transverse to the axis of the drift tube. A drift gas which flows through the drift tube at right angles to the electric fields transports the ions along the axis of the drift tube.
An ion mobility spectrometer of the time-of-flight type has a pneumatic system which usually comprises the drift tube, a gas pump (gas transport system), filters and gas channels (gas connections). The gas channels connect the pneumatic components (drift tube, gas pump and filters) to form a gas circuit in which the gas is circulated by the gas pump. In order to achieve reproducible results with an ion mobility spectrometer of the time-of-flight type, in particular, the gas must be continuously cleaned and the moisture kept as constant as possible at a low level (10 to 100 parts per million). The filters clean the circulated gas by removing the substances to be analyzed which have not been ionized in the reaction region; and they extract excess moisture brought into the gas circuit by the gas sample. Inside the drift tube, the ions encounter a constant flow of gas which is thoroughly cleaned to prevent the formation of additional ions of the substances to be analyzed. To control the gas flow, the pneumatic system can also have pneumatic switching and control elements, e.g. switching valves. The sensors of the ion mobility spectrometer monitor important gas parameters such as the pressure, temperature and moisture.
Along the drift path of the drift tube an axial electric drift field has to be generated. In order to minimize any temporal broadening of the measured ion current pulse for an ionic species, the drift field should have maximum homogeneity transverse to the axis of the drift tube. A homogeneous axial electric drift field can be generated by electrically conductive ring electrodes, for example, which are arranged so as to be equidistant and concentric to the axis of the drift tube, and across which uniformly increasing electric potentials are applied (Eiceman U.S. Pat. No. 4,777,363). The field homogeneity improves with increasing ratio of the inner diameter to the separation distance of the ring electrodes. The individual potentials are usually tapped from a shared voltage divider.
In most cases, the reaction region of the drift tube is constructed like the drift region, although here the field homogeneity does not have to be as high and hence a larger separation distance or a smaller diameter of the ring electrodes can be chosen. Besides the ring electrodes, the drift tube contains a gating mechanism between the reaction region and the drift region as well as a screen grid near to the collecting electrode. All electrically conductive parts of the tube, including the ionization source located in the reaction region, must have a defined electric potential and therefore must be contactable from outside. These electrically conductive parts are termed “electrodes” below.
The drift tube of an ion mobility spectrometer of the time-of-flight type must customarily perform the following tasks:                Sealing off the drift region from the outer region of the drift tube,        Ionizing the gas sample in the reaction region,        Generating a homogeneous axial electric drift field, particularly along the drift path,        Mechanically holding the electrodes and their electrical insulation,        Preventing the gas sample from being carried into the drift path.        
To prevent an unfiltered gas sample from being carried into the drift path, the drift path is purged with cleaned drift gas, which flows from the collecting electrode to the gating mechanism and keeps the gas sample away from the drift path. The gas stream in the drift tube ensures that any outgassings which may be present are continuously flushed off the interior walls of the drift tube. It is also a requirement for the drift tube that outgassings from the drift tube itself are minimized.
An early version of a drift tube of the time-of-flight type has a support in the interior of a gas-tight container; the support mechanically holds the electrodes of the drift tube (especially the ring electrodes) and electrically insulates them from each other (Cohen et al. U.S. Pat. No. 3,699,333). The drift tube is connected to the other components of the pneumatic system via hoses or metal pipes. The electrical connection of the electrodes is made via electric feedthroughs in the gas-tight container. In order to minimize the number of expensive electrical feedthroughs, a voltage divider which provides the individual potentials for the ring electrodes can be installed in the interior of the gas-tight container. A similar version (Kyoung et al. U.S. Pat. No. 5,834,771) realizes the ring electrodes with the aid of a flexible circuit board providing metallized strips, bent into a tube, and likewise housed inside a gas-tight container.
The separate realization of the mechanical electrode holder and the sealing of the drift tube leads to a relatively large arrangement which is not particularly suitable for small, mobile instruments. Moreover, the gas-tight container must have a relatively large aperture, which must be subsequently closed, so that the support can be mounted. Furthermore, either there has to be a large number of electrical feedthroughs in the gas-tight container, or the electrical components in the interior of the drift tube, e.g. the resistors of the voltage divider or the flexible circuit board, have to be made of a material which does not outgas. Both of these are very expensive.
In another version, the ring electrodes are replaced by tubes or tube segments which are weakly electrically conductive or whose inside has a coating with low electrical conductivity (Browning et al. U.S. Pat. No. 4,390,784). A gas-tight container can be used to hold and center the tubes or tube segments (Vora et al. U.S. Pat. No. 4,712,008). This simplifies the mechanical set-up and reduces the number of electrodes for which contacts have to be made. Tube segments with an interior coating still require a certain number of electrical feedthroughs, however.
Campbell et al. (U.S. Pat. No. 5,021,654) use a monolithic ceramic block which has a coating with low electrical conductivity in the interior, and to which all other components of the drift tube are mechanically fastened. Numerous electrical contacts are fed through the wall of the ceramic block here, and it is very labor-intensive to seal them all. Moreover, the monolithic ceramic block cannot be manufactured by a method that is suitable for mass production, such as dry pressing followed by sintering, because the block has a complicated design with a large number of apertures which lead in different directions. In U.S. Pat. No. 5,021,654 a glass ceramic which can be machined in the fired state is used for the monolithic ceramic block. However, the material's porosity and high surface affinity for water make it unsuitable for an ion mobility spectrometer, which is operated under changing climatic conditions. The machining of other ceramic materials which are suitable for ion mobility spectrometers is extremely difficult in the fired state because they are so hard and brittle. Moreover, the reproducible and homogeneous coating of parts of the inside of the ceramic block with a weakly conducting material is very demanding technologically so that this design is not suitable for mass production.
Another version uses tubes which are not themselves electrically conductive but which have conductive ring electrodes on the outside or a continuous coating with low electrical conductivity (Burke U.S. Pat. No. 5,162,649; Vandrish et al. EP 0 369 751; Kaltschmidt et al. DE 197 27 122; Leon EP 0 505 216). With this version, the electrodes of the drift tube can be provided with electrical contacts from the outside without the need for expensive feedthroughs. The electric field penetrates into the interior either capacitively or as a result of leakage currents in the tube.
To monitor the ambient air for pollutants, the ions of both polarities are usually detected, a process which is effected by cyclically changing over the direction of the electric drift field at intervals of a few seconds. With the drift tubes just described, ions which reach the insulating inner wall of the tube are not neutralized but remain there as charged stationary ions and bring about a static charging of the inner wall. This charge focuses subsequent ions of the same polarity at the axis of the drift tube, so the ion current at the collecting electrode increases. If the direction of the electric drift field is changed over in order to measure ions of the other polarity, these same charges initially bring about a deflection of the ions to be detected to the inner wall of the drift tube, thus reducing the measurement signal. The signal increases until the charge reversal of the inner walls is complete. This charge reversal process takes between a few minutes and hours to stabilize and thus prevents ions of both polarities from being measured quickly. Externally coated tubes are hence only suitable for special measuring tasks where it is sufficient to detect ions of one polarity.
In another version, the drift tube is assembled from electrically conductive and insulating rings. In the direction of the axis of the drift tube, electrically conductive and insulating rings alternate. The electrically conductive rings (ring electrodes) extend from the interior of the drift tube to its exterior surface so that electrical contact can be made with each ring electrode from the outside (Knorr et al. U.S. Pat. No. 4,633,083; Eiceman U.S. Pat. No. 4,777,363; Avida et al. U.S. Pat. No. 5,235,182). The contact surfaces between the individual rings are sealed. One production method for such a drift tube consists in tightening the stacked rings mechanically from the outside by inserting sealing rings. The mechanical support required for this is an additional expense, however, and requires space, which is at a premium in small mobile instruments. Furthermore, the large number of sealed points presents a high risk of leaks. A simpler drift tube with significantly greater mechanical robustness is obtained by using metallic ring electrodes with Z-shaped cross section into which perfectly fitting insulating rings with rectangular cross section are inserted. The stacked ring electrodes and insulating rings are bonded or soldered (Karl DE 41 30 810); this creates a self-supporting, gas-tight drift tube which can be equipped with electrical contacts from the outside. Unavoidable assembly tolerances when assembling the stacked ring electrodes and insulating rings with this production method give rise to relatively large length tolerances for the finished drift tube when shrinkage of the bonded layers and soldered layers occurs.
With all types of ion mobility spectrometer up to now, the drift tube is usually connected to the pneumatic components of the ion mobility spectrometer via separate gas pipes (gas channels), such as hoses or capillaries. Producing and assembling the gas pipes of the ion mobility spectrometer represents a large proportion of the overall manufacturing cost.
In the case of the drift tubes assembled from ring electrodes and insulating rings, in particular, the gas pipes generally have to be flexible to compensate for the relatively high tolerances of the assembly and joints. To mount the flexible gas pipes, suitable adapters are integrated into the drift tube, creating more costs. The use of flexible hoses is not ideal, especially for gas analysis, as they represent an increased risk of leaks and outgassing. Moreover, impurities can enter the drift tube when hoses are being fitted, requiring additional cleaning. Compensating for the tolerance with curved and soldered metal capillaries promises better tightness, but is more expensive to manufacture and assemble, and requires additional work procedures to clean flux residues from the joints and the drift tube. Alternatively, the bonded or soldered drift tubes can be made to fit perfectly by mechanically reworking the drift tubes in the assembled state. However, clamping and aligning the drift tube is time-consuming. Moreover, additional cleaning is again necessary to remove pollutions introduced by machining.